Method of heat-treating workpieces



United States Patent US. Cl. 148--13.1 Int. Cl. 021a 1/44 4 Claims This invention relates to the protection of the surface of metal workpieces whilst these are being heated to elevated temperatures, for instance during heat treatment.

Such workpieces include sintered hard materials having a steel matrix, which are hardenable by austenite decomposition or precipitation, and which have a high content of carbides, particularly of titanium carbide and tungsten carbide. Tool steels and high speed steels of conventional composition are also included as measures must be taken with such materials to ensure that their surfaces are neither significantly decarburised nor carburised during such treatments.

The necessity of providing complete surface protection is particularly important with the above-mentioned hardenable hard materials, as they have the useful property of only slightly changing in volume when for instance they are hardened. As they are not therefore liable to distort, they can be finish-machined before they are hardened whilst they are still relatively soft and therefore machinable.

In these circumstances, the workpieces, such as for example finished machined tools, must be heated to hardening temperature in such a way that oxygen cannot gain access to their surfaces and cause decarburisation, and recarburisation must also be prevented.

In the hardening of steel, the use of neutral salt baths, vacuum furnaces, furnaces filled with inert protective gases and so forth is already known, and application of these techniques to the heat treatment of sintered hardenable steel is possible. However, there is a strong demand for a method which dispenses with such relatively expensive devices, and allows the heat treatment to be carried out in a conventional electrically or gas heated furnace.

In the art of heat treating steels, particularly tool steels and high speed steels, packing substances have been developed for embedding such steels. However, these have the disadvantage that they cannot completely ensure the provision of a neutral environment. Completely burnt-out charcoal may be used as a packing material but this substance must be applied in the form of a thick packing to prevent the penetration of oxygen, but even so recarburisation cannot be thus always prevented. The same difficulty applies when burnt-out coke fines are used and cast iron chips. When packings of oxidic materials are used, decarburised surface zones form and give rise to a soft external skin. Another known method consists in using mixtures of coke fines, charcoal and oxides, such as silicon dioxide (sand), to provide a thoroughly neutral mixture. Although neutral mixtures may thus be successfully formed, there is still no absolute certainty because the state of neutrality changes when the mixture has been used once. Another proposal for protecting tool steels and high speed steels when these undergo thermal treatments, consist in packing them in chips of the treated materials themselves. However, even such chips can at best be used only once, and they may not then be completely effective, which procedure is therefore expensive.

Coating pastes based on water glass are likewise known. However, the water contained in the water glass oxidises the workpieces, and renders these pastes useless. Other coating pastes have been proposed which when heated to about 900 C. form a liquid coating of boron silicate on "ice the workpiece surfaces. The application of such a paste is a nuisance in that lengthy periods are needed for drying the paste, and the coating is afterwards diflicult to remove when the workpiece has been hardened. Such pastes do not have an adequate protective effect.

According to the invention surface damage to metal workpieces as hereinbefore described by oxidation or carburisation during heat-treatment can be prevented by using as a packing material a comminuted sintered hard material of the following composition:

Percent by weight Titanium carbide 30-40 Chromium 1-15 Molybdenum 1-5 Carbon 0.2-0.8

Remainder, iron (impurities and incidental ingredients included).

A hardenable sintered hard material of the kind proposed as the packing material is known per se. It may be used in the form of chips, produced when a sintered workpiece of the said alloy is worked, e.g. when formed, planed or milled. It is advisable to reduce these chips in ball mills or the like to a grain size under 0.1 mm., as a coarser grain size will not stop oxygen from penetrating to the surface of the workpieces. The metal workpieces to be treated according to the invention may be packed in boxes and immersed in the size-reduced packing material. Heating is done in conventional furnaces without a protective gas atmosphere, and temperatures between 950 and 1100 C. may be reached. These temperatures are usually maintained for at least 20 minutes to ensure that the workpieces are thoroughly heat-soaked, quenching being then carried out in oil or in a warm bath, the tools or other articles including the powder being simply tipped into the bath or, if the workpieces are of large size, they may first be taken out of the powder and then immersed in the bath.

A particularly effective hard material for use as packing, according to the invention, has the following composition:

Percent by weight Titanium carbide about 33 Chromium do 3 Molybdenum do 3 Carbon -do 0.33

Remainder iron (impurities and incidental ingredients included).

The packing material according to the invention has the special advantage of being substantially neutral. Whereas previously-known packing substances or coating pastes gave rise to zones of influence on the surface of the workpieces which may be p. thick, and even neutral salt baths create zones of influence about 10a thick, the powder proposed by the present invention will ensure that the zones of influence are not more than 5 Furthermore, they afford the surprising advantage that the powder can be repeatedly used without any appreciable reduction in its eflectiveness. It has been ascertained that even after the powder had been used 10 times in succession, it still has the property of absorbing substantial volumes of oxygen, and thus to keep the oxygen away from the surface. For example, a powder of composition according to the invention had an initial oxygen content of 0.45%. After this powder had been used 15 times, the oxygen content was 13%, and after it had been used 20 times the oxygen content was 20%. The repeated applicability of the powder thus ensures a high degree of economy. Furthermore, it is also possible to extend the powder by adding other substances conventionally used as packing materials, such as burnt-out coke fines, burnt-out charcoal, quartz meal or the like. However, such added matter should preferably not exceed by weight, since otherwise the neutrality of the powder may not be preserved. On no account should the addition exceed 50% by weight.

Examples of hardenable sintered hard materials which are particularly useful as packing media during thermal treatments, according to the invention, may comprise the components hereunder listed, although it will be understood that the individual contents may slightly deviate from the specified figures upwards or downwards:

Percent Carbon 0.06 Titanium carbide 32 Chromium 12 Nickel 8 Titanium 0.5 Carbon 0.4 Titanium carbide 32 Chromium 14 Carbon 0.35 Titanium carbide 33 Chromium 2 Molybdenum 2 Copper 2 Carbon 0.35 Titanium carbide 33 Chromium 2 Molybdenum 2 Carbon 0.3 Titanium carbide 11 Tungsten carbide 34 Vanadium carbide l Chromium carbide 3 Molybdenum 0.5

The proposed pulverulent protective media may also be used whilst the sintered hardenable hard material alloys being treated are being normalised and soft annealed, if these treatments are to be performed in an ordinary air furnace. Such thermal treatments are needed after sintering in order to provide the material with the best type of structure for machining. It is thus possible to avoid performing the heat treatment in a vacuum sinter furnace, which thus becomes available for other work.

What is claimed is:

1. A method of heat-treating hardenable metal workpieces which comprises packing the metal workpieces in a size-reduced sintered hard material composition com prising Percent by weight Titanium carbide 30-40 Chromium 1-15 Molybdenum 1-5 Carbon 0.2-0.8

Remainder, iron.

and heat-treating the packed metal workpieces.

2. The method of claim 1, in which the size-reduced sintered hard material contains in addition to the metal constituents thereof, up to by weight, based on the said metal constituents of inorganic matter of the group consisting of burnt-out coke fines, burnt-out charcoal or quartz meal.

3. The method of claim 2, in which the said inorganic matter is present in proportions of not more than 20% by weight, based on the said metal constituents.

4. The method as claimed in claim 1, in which the said size-reduced sintered hard material comprises Percent by weight Titanium carbide 33 Chromium 3 Molybdenum 3 Carbon 0.35 Remainder iron.

References Cited UNITED STATES PATENTS 604,195 5/1898 Rogers 148-14 X 2,040,370 5/1936 Fisher 148--14 2,752,666 7/1956 Goetzel et al. 29-182.8

2,828,202 3/1958 Goetzel et al. -123 3,380,861 4/1968 Frehn 148-126 CHARLES N. LOVELL, Primary Examiner.

U5. (:1. X.R. 14s 14, 27 

1. A METHOD OF HEAT-TREATING HARDENABLE METAL WORKPIECES WHICH COMPRISES PACKING THE METAL WORKPIECES IN A SIZE-REDUCED SINTERED HARD MATERIAL COMPOSITION COMPRISING 